function check_form() {
	 var filter=
			/^([\w-]+(?:\.[\w-]+)*)@((?:[\w-]+\.)*\w[\w-]{0,66})\.([a-z]{2,6}(?:\.[a-z]{2})?)$/i ;
			
 	if (document.main_form.name.value == '') { 
		alert('Please, write your name.'); 		
		return false; 
	}
 	if (document.main_form.last_name.value == '') { 
		alert('Please, write your surname.'); 
		return false; 
	}
 	if (document.main_form.email_address.value == '') { 
		alert('Please, enter your e-mail adress.'); 
		return false; 
	}
	if (!filter.test(document.main_form.email_address.value)){
   		alert("Please, enter a valid email account.");
		document.main_form.email_address.value = "";
		return false;

	}
 	return true;
	
 }